Enhancement: Move key add, list, remove and passwd as separate sub-commands

Restic now provides usage documentation for the `key` command. Each sub-command;
`add`, `list`, `remove` and `passwd` now have their own sub-command documentation
which can be invoked using `restic key <add|list|remove|passwd> --help`.

https://github.com/restic/restic/issues/4676
https://github.com/restic/restic/pull/4685
